Although PHP 5 offers many new features, it's designed to be as
compatible with earlier versions of PHP as possible with little
functionality being broken in the process.

Be sure to read the appropriate PHP 5
migration appendix of this manual as it contains even more
information on the topic of migrating to PHP 5.

Does MySQL work in PHP 5? It seemed to have disappeared.

MySQL is supported with the only
change being that MySQL support is no longer enabled by
default in PHP 5. This essentially means that
PHP doesn't include the --with-mysql
option in the configure line so
that you must now manually do this when compiling PHP. Windows users will
edit php.ini and enable the php_mysql.dll DLL as
in PHP 4 no such DLL existed, it was simply built into your Windows
PHP binaries.

Also, the MySQL client libraries are no longer bundled with PHP. More
details on this topic are covered in
the following FAQ and
be sure to read the MySQL section
for details on installing MySQL. An example configure line would be
--with-mysql=/usr while Windows users
will need the libmySQL.dll available to the system.

I hear PHP 5 has an entirely new OOP model, will my existing OOP code
work? Where do I find information on these new OOP features?

The main change in PHP 5 is to the OOP model as PHP 5 now uses
the Zend Engine 2.0. The
zend.ze1_compatibility_mode directive enables compatability
with the Zend Engine 1.0 (PHP 4).